Historically, telephony networks have been designed around “dumb” endpoints that require all services to be provided from inside the network. This assumption shaped the architectural design of today's telephone networks and the technology used therein. With the advent of packet-based telephony, however, the world is about to undergo a dramatic and disruptive change. This change is caused by the fact that VoIP protocols like the Session Initiation Protocol (SIP) adhere to the “end-to-end” principle, which suggests that the network should be kept as simple as possible and that all intelligence should reside in the end systems. When applied to telephony networks, this means that a VoIP-based telephone infrastructure will bring about a paradigm shift from a network-centric to an endpoint-centric design.
Endpoint vendors are taking advantage of this fact by deploying intelligent VoIP phones, which provide services that were formerly hosted inside the network. For example SIP phones are available that support features such as call forwarding, call transfer, three-way conferencing, and call logging. Other phones are available, which support a Java-based runtime environment, that can host even more services and applications. Even further, a full-featured SIP endpoint terminal can be integrated into the Windows operating system, turning every PC into a sophisticated, easily programmable phone.
Disadvantageously, these VoIP endpoint-provided services cannot be provided as reliably as comparable services can currently be provided from within a PSTN network. In PSTN networks, telecom services such as call forwarding, call logging, teleconferencing, and voice mail, are typically provided by entities located within the service provider's network, such as at application servers or Service Control Points (SCPs). Continuous availability of these network-hosted services is maintained through the use of highly reliable hardware and software. In the event of a server or software failure, a secondary server typically takes over so that service can continue uninterrupted. When the provision of such services is shifted to intelligent VoIP endpoint terminals, which have more processing power than typical PSTN telephone terminals, their increased complexity makes them much less reliable than typical PSTN telephones. For example VoIP endpoint terminals may be switched off, crash, loose power, or in the case or wireless endpoint terminals, be out of reach. Thus, the endpoint terminal-provided services that the end user would assume to always be available would not be available when the endpoint terminal itself is unavailable for any reason.